<> == How can I determine whether a command exists anywhere in my PATH? == POSIX specifies a shell builtins called `command` and `type` which can be used for this purpose. Note that type's exit codes isn't well defined by POSIX whereas command's exit status is well defined by POSIX, so that one is probably the safest to use. {{{ # POSIX if command -v qwerty > /dev/null; then echo qwerty exists else echo qwerty does not exist fi # POSIX (no options in `type`) if type qwerty > /dev/null; then echo qwerty exists else echo qwerty does not exist fi }}} In BASH, there is `hash` builtin and `type` with non-POSIX options. Here's are examples: {{{ # Bash if hash qwerty 2> /dev/null; then echo qwerty exists else echo qwerty does not exist fi # Bash # type -P forces a PATH search # skipping builtins and so on if type -P qwerty >/dev/null; then echo qwerty exists else echo qwerty does not exist fi }}} KornShell and zsh have `whence` instead: {{{ # ksh/zsh if whence -p qwerty > /dev/null; then echo qwerty exists else echo qwerty does not exist fi }}} The `command` builtin also returns true for shell builtins (unlike `type -P`). If you absolutely must check only PATH, the only POSIX way is to iterate over it: {{{ # POSIX IsInPath () ( [ "$1" ] || exit 2 set -f; IFS=: for dir in $PATH$IFS; do [ -x "${dir:-.}/$1" ] && exit 0 done exit 1 ) if IsInPath qwerty; then echo qwerty exists else echo qwerty does not exist fi }}} Note that the function defined above uses parentheses around the body rather than the normal curly braces. This makes the body run in a subshell, and is the reason we don't need to undo `set -f` or [[IFS]]. The iterative approach is also used in `configure` scripts. Here's a ''simplified'' version of such a test: {{{ # Bourne save_IFS=$IFS IFS=: found=no for dir in $PATH; do if test -x "$dir/qwerty"; then echo "qwerty is installed (in $dir)" found=yes break fi done IFS=$save_IFS if test "$found" = no; then echo "qwerty is not installed" fi }}} Real `configure` scripts are generally much more complicated than this, since they may deal with systems where `$PATH` is not delimited by colons; or systems where executable programs may have optional extensions like `.EXE`; or `$PATH` variables that have the current working directory included in them as an empty string; etc. If you're interested in such things, I suggest reading an actual GNU autoconf-generated `configure` script. They're far too large and complicated to include in this FAQ. The command `which` (which is often a csh script, although sometimes a compiled binary) is '''not reliable''' for this purpose. `which` may not set a useful exit code, and it may not even write errors to stderr. Therefore, in order to have a prayer of successfully using it, one must parse its output (wherever that output may be written). {{{ # Bourne. Last resort -- using which(1) tmpval=`LC_ALL=C which qwerty 2>&1` if test "$?" -ne 0; then # FOR NOW, we'll assume that if this machine's which(1) sets a nonzero # exit status, that it actually failed. I've yet to see any case where # which(1) sets an erroneous failure -- just erroneous "successes". echo "qwerty is not installed. Please install it." else # which returned 0, but that doesn't mean it succeeded. Look for known error strings. case "$tmpval" in *no\ *\ in\ *|*not\ found*|'') echo "qwerty is not installed. Please install it." ;; *) echo "Congratulations -- it seems you have qwerty (in $tmpval)." ;; esac fi }}} Note that `which(1)`'s output when a command is not found is ''not'' consistent across platforms. On HP-UX 10.20, for example, it prints {{{no qwerty in /path /path /path ...}}}; on OpenBSD 4.1, it prints {{{qwerty: Command not found.}}}; on Debian (3.1 through 5.0 at least) and SuSE, it prints nothing at all; on Red Hat 5.2, it prints {{{which: no qwerty in (/path:/path:...)}}}; on Red Hat 6.2, it writes the same message, but on standard error instead of standard output; and on Gentoo, it writes something on stderr. Use one of the builtins or the iterative approaches is recommended instead of `which`.